In continuous casting plants for slabs, to reduce the core porosity and the core segregation, the soft reduction process (SR process), among others, is used.
In bloom, ingot and billet plants, the soft reduction is preferably effected in a straight-drive region. The precondition for an improvement of the internal structure quality is that the final solidification of the strand be effected in the soft reduction (SR) stretch which is set to be tapered or conical. A final solidification upstream or downstream of the SR stretch not only does not give rise to any improvement in the internal quality, but on occasion can result in a detriment to the internal quality.
The conical or tapered roll setting of the strand guide segments is effected either as a fixed setting by means of hydraulic cylinders and spacer members or as a flexible setting, for example with position-controlled hydraulic cylinders.
The so-called operating window of the continuous casting plant, for example the matching of the casting speed or another casting parameter, the intensity of the spray cooling or the steel quality is given by the number of SR roll pairs and the type of adjustment. With an increasing roll count, the operating window can be increased. This increase however is not in proportion to the increase in the capital cost. Furthermore the position of the sump tip or melt-cavity tip can only be influenced to a limited extent by the spray cooling.
In a known process according to the German Patent Document DE 41 38 740 A1, the strand can pass through a soft reduction stretch at whose entry the strand is not yet fully solidified but at whose end the strand is solidified throughout its thickness and for which, among other things, the casting speed is a significant operating parameter. In the region of final solidification, a thickness reduction is effected, for example for thin slabs, of between 0.5 and 3 mm per meter of the cast length. For this purpose in the soft reduction stretch, the roll pairs of individual segments are brought closer together to follow the shrinkage ratio of the strand so as to provide a compaction of the internal or lattice structure in the region of residual solidification and to bring about an improvement in the strand internal quality.
European Patent Document EP 0 834 364 A1 describes a method and a device for high speed continuous casting plants with a strand thickness reduction during the solidification, whereby after the so-called cast rolling, the strand cross section is linearly reduced over a minimum length of the strand travel directly below the mold, this being followed by further strand reduction over the remainder of the strand path, the “soft reduction” until immediately upstream of the final solidification or the sump or melt-cavity tip. Through these technological features of the method, the strand cross section reduction is so carried out that a critical deformation of the strand shell is not exceeded in consideration of the high casting speed and steel quality.
European Patent Document EP 0 17 77 96 B1 discloses a method of advancing and dressing straightening a cast strand in the straightening and outlet region of an arcuate continuous casting plant, whereby oppositely disposed rolls are biased by a spring force against the ferrostatic pressure of the cast strand and are adjustably spaced from one another during the casting process. In this manner a cast strand region of increased solidity is avoided and during the travel of the strand between opposing rolls the respective spring force can be reduced by a lower counter force.
The slabs or blooms produced in the continuous casting plants corresponding to the aforementioned documents serve as starting materials for rolling milled products for the production of plates or strip.